Regulatory Requirements - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($)
Regulatory Capital Requirements [Abstract]
Minimum period required for financial resources to cover operating costs	1 year
Minimum period required for cash or highly liquid securities to cover operating costs	6 months
Net assets held by regulated subsidiaries	$ 530.7
Amount of capital in excess of aggregate regulatory requirements	$ 284.5